I kicked around an idea of 3D printing a compressible fine aluminum mesh, shoving it into a hybrid rocket shell, then heat the shell casing to wax melting temperatures (but below aluminum) and slowly poor wax into the mesh structure through the central borehole of a fuel grain. The mesh provides structural reinforcement to the wax plus raw aluminum for combustion. It would also provide some heat transfer mechanisms, like the zirconium staples in the Sprint ABM solid rocket motor.

The Sprint ABM did this in the 60's using what was described as a fairly primitive robot arm to position layers of "caltraps" made from either Aluminum or Magnesium in the double base propellant. Experimental motors got a recession rate up to 50 inches a second (unless they turned from a deflagration into a detonation in which case they suffered RUD ).

In the same way that solids are basically a technology for weapons (long storage times, higher Isp than hybrids,quite dangerous to handle) hybrids are basically a technology for civilian systems (easyish to make and mover around but lower Isp).

So this could be done without 3d printing but the where 3d could score is making a more rigid fuel structure that's less prone to both causing vibration and more resistant to its effects (IE not spalling off big chunks that can get trapped in a nozzle).

However that implies that the combustion effects that cause vibration are well enough understood to devise ways to control them. I'm not sure if they are.

This also suggests you're looking at large scale 3d printing, not at the small parts level.

And as Scaled Composites discovered during the Spaceship Two development work while Nitrous Oxide is relatively safe to handle it's pretty dangerous when mis-handled. But there is probably more experience in its use through car NoX systems than there is of HTP.

Time will tell if they can do better than American Rocket in this regard. I wish them luck.

Logged

"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11 Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly. STS-Keeping most of the stakeholders happy most of the time.